EP3002128B1 - Method for printing on a curved surface of an object with an ink jet head - Google Patents

Description

The present invention relates to a method having the features of the preamble of claim 1.
The invention is in the technical field of ink jet printing, in particular the printing of non-planar, but curved substrates. "Curved" means that the surface has convex and / or concave sections, etc., such as body panels of vehicles.
The known state of the art in this technical field comprises the following:
From the DE 10 2012 006 371 A1 There are already known a method and a device which allow to print on curved surfaces and to dry the print. An ink jet head and a dryer are guided along the object by means of a robot or vice versa. The not yet published DE 10 2013 016 006.5 and DE 10 2014 004 507.2 disclose method and apparatus which in particular allow to print spherical objects, whereby the respective object is moved.
From the not yet published DE 10 2013 014 444.2 For example, a method is known which allows to plan webs for printing and drying curved surfaces and to guide the printhead / dryer on the intended tracks.
The DE 10 2012 006 370 A1 and the not yet published DE 10 2013 019 359.1 Disclosed methods and apparatus that allow to plan or execute multiple parallel paths when printing on curved surfaces such that noticeable disturbances in the lateral connection of the printed ink traces are avoided.

The US 2014/0132673 A1 discloses a printing system and corresponding printing method in which an inkjet printhead is moved in mutually perpendicular paths over a substrate. At the end of a first lane and at the beginning of a second lane these overlap. The document discloses various ways of printing nozzles on which tracks in the overlap areas. The document also describes rotating the printhead so that it is always oriented perpendicular to the forward direction. The area to be printed is primarily composed of non-overlapping webs.

The WO 2014/060005 A1 describes a very similar printing system or method in which a rotatable printhead is moved along angularly spaced tracks over a substrate to be printed. Again, the area to be printed is composed essentially of non-overlapping web sections. Only in the corners of the section to be printed overlap the webs.

The US 6,176,961 B1 describes a device for simultaneously applying adhesive to a plurality of juxtaposed objects. In this case, according to one embodiment, a single head is moved on a single track or, according to another embodiment, two heads are moved on two tracks. The sinusoidal paths generated thereby overlap at points of intersection. However, the adhesive portion, which extends over a plurality of individual objects, is composed essentially of areas in which the adhesive tracks do not overlap.

If the print has to be dried, it may be necessary to print short prints in order to dry them without disturbing time delay. However, a plurality of short tracks also create a plurality of terminals of the tracks to each other.

This can also increase the likelihood of noticeable disturbances.
It is also known that the human eye perceives long and straight lines in the otherwise homogeneous printed image easier than short and running in curves, Against this background, it is an object of the present invention to provide a comparison with the prior art improved method which it allows to create at least two printed inkjet tracks whose connection to each other is imperceptible to the naked eye and therefore has no interference.
An inventive solution to this problem is a method having the features of main claim 1.

The invention describes a method according to claim 1.

According to the invention, the angle between the track edges is not 0 ° and also not 180 °, ie the two printed tracks are not parallel to each other. Preferably, the angle is not 90 ° or not in the range between about 85 ° and 95 °, ie the two printed tracks are not perpendicular to each other. The method according to the invention therefore advantageously makes it possible to produce two printed inkjet tracks whose connection to one another is imperceptible to the naked eye and therefore has no disturbances.

The angle α can preferably be selected such that a = arctan (n * b / m * 1) with α ≠ 90 °. Where: b = distance between two adjacent nozzles of a print head, 1 = distance between two consecutive pressure points in the direction of movement of the print head, n, m = natural numbers. This ensures that the respective last pressure point of a pressure point row of a second track has the same distance from the pressure points of the first track adjacent to the second track, so that a homogeneous transitional area is created between the two tracks.

If the image to be printed and composed of the tracks is screened, it may be advantageous to choose the screen angles within two tracks so that they are matched to the angle between the track edges of the two tracks and the occurrence of noticeable connections of the tracks to each other reduce.

The tracks may be approximately straight along the curved surface. However, they can also have curves. The width of the tracks may be substantially constant. However, the width may also change, e.g. can decrease the width by turning the print head or by turning off nozzles on the edge of the head.

It may also be advantageous to take into account the angles between track edges and possible overlap areas used in path planning already during the generation of the print data (in the so-called RIP). Preferably, such that each pixel is printed only once, even with repeated passing over the print head. Pressure points in the respective area of the connection of two tracks are preferably assigned to one of the two tracks in the RIP.

The track edges can preferably be printed expiring and mesh with adjacent track edges (so-called stitching).

The inventive method described can advantageously lead to a lack of mechatronic precision of a robot-controlled ink jet head not or only to a lesser extent perceptible, because spurious lines are avoided, i. longer parallel pressure gaps with reduced optical density or print overlaps with increased optical density. Such lines perceive the human eye much more sensitive than mutually angled deviations of the pressure points of a given ideal grid.

When driving through an articulated-arm robot (as a movement unit for the print head) through the theoretically achievable space on predetermined paths, so-called singularities occur. These can not be traversed by the robot in practice. As a result, individual planned tracks could not be printed and the practically usable space of the robot would be restricted. However, singularities can be avoided if the trajectory passing through this point of singularity can be changed. According to the invention, a robot which moves the print head can vary the printing webs at least in the image plane, since the webs no longer necessarily have to be parallel. This reduces the number of singularities and increases the space in which the robot can print. In other words, by the non-parallel web guide larger objects can be printed with the same robot, as in the case of parallel web guide.

Advantageous and therefore preferred developments of the invention will become apparent from the accompanying dependent claims and from the description and the accompanying drawings.

A preferred embodiment of the method according to the invention may be characterized in that the angle is between about 20 ° and about 70 ° or between about 110 ° and about 160 °, preferably about 45 ° or about 135 °.

In the overlap area, the image area to be printed can be generated when printing a track. Alternatively, the image area can be created when printing two or more tracks. With two tracks, each track can contribute about half of the pixels in the image area, with three tracks each contributing about one third.

A preferred development of the method according to the invention can be distinguished by the fact that the inkjet head on the first web and / or on the second web changes its orientation relative to the respective web.

A preferred development of the method according to the invention can be distinguished by the fact that the inkjet head on the first web and / or on the second web changes its orientation relative to the respective web. This process can be referred to as a so-called "tilted pass".

A preferred embodiment of the method according to the invention can be characterized in that the inkjet head is rotated during its forward movement on the first path and / or on the second path about an axis, in particular about an axis perpendicular to the first track or second track axis.

With the simple tilt pass, the angle of the printhead is changed to the printing direction. This changes the maximum print width of the print head, but the print dot density of the track to be printed is kept constant by compensating the print gaps of the individual print dot rows. It is also a multiple Tilted Pass possible. This creates at least one overlapping area whose print image is composed of pixels multiple tracks.

With the Dynamic Tilted Pass, the track to be printed - especially at the beginning or at the end of the track - can become narrower during the printing of a track and can even end in a (print) point. The boundary or the track edge may be curved. The subsequent track can also begin narrower (or in this extreme case at this point), so that the connection of two tracks can be reduced to one point. This results in tracks that (partially) have no parallel boundaries. The lateral connection is preferably realized by a track with a likewise curved boundary.

Instead of a preferably curved surface can be printed with the inventive method, a flat or flat surface. It can preferably be provided that the tracks and associated tracks or track edges angle of about 120 ° to each other. Such printing may be referred to as "hexagonal printing" because of the resulting tri or hexagonal structures. Such flat substrates may preferably be substrates that are in an upright orientation, eg billboards or other billboards, flat sections of building facades or room walls, flat sections of sidewalls of vehicles (trucks, trailers, containers, trains, wagons), traffic or signs.

It can also be provided to store the calculated pressure points of a first path planning instead of printing as a mosaic. These data serve as information for a second path planning, the second path planning being different from the first path planning, i. has other trajectories. The actual printing takes place after this second path planning as a mosaic. This method should z. B. reduce moiré-like effects. (Edge) pressure points, which lie only proportionally on a web of the second path planning, are printed in a correspondingly proportionate size or as a gray value. The traces of the second mosaic can optionally be created with the printhead tilted. The pressure point rows of these tracks are then closer together, the maximum pressure point density is correspondingly higher. As a result, the theoretical pressure points of the first mosaic can be better or more accurately reproduced.

The total print image to be generated may be mosaically composed of the tracks, with any track shapes being printable. It is noted that the track shapes fill the printed image in the manner of repetitive or non-repetitive tiling.

The invention is not only applicable to the printing of a color, but also in the multi-color printing, eg CMYK printing. In this case, each color separation can be treated separately according to the invention, or the inventive method and its developments and the corresponding path planning can be performed separately for each color. It is particularly advantageous if the track edges of two or more mutually different colors are not adjacent to each other or parallel, but according to the invention include an angle different from 0 ° and 180 °. A corresponding method can proceed as follows: First, a first Color printed on the object, wherein the track edges of the first color include angles between about 1 ° and about 179 ° to each other, preferably between about 5 ° and about 175 °. Then the first color is pinned, ie dried, but not completely dried, or partially cured. Then a second color is printed, with attention also being paid to angles between about 1 ° and about 179 ° to each other, preferably between about 5 ° and about 175 °. The track edges of the second color are aligned so that they not only to each other, but also to the track edges of the first color have said angles. Accordingly, further colors are used, always making sure that all track edges to each other have the said angular relationships. Any color except the last color, if any, will be pinned. Finally, all printed colors are completely dried or hardened together.

An alternative mechanical solution in multi-color printing may provide that the printheads for the individual colors are arranged below said angular relationship, e.g. are mounted on a robot arm. In this case, a single path planning would be sufficient for all colors, since the heads when moving the robot to each other have different (fixed) angular relationships and print corresponding "angled" tracks.

The invention as such and advantageous developments of the invention are described below with reference to the accompanying drawings with reference to two preferred embodiments. In the drawings, corresponding elements are provided with the same reference numerals.

Figur 1

eine schematische Perspektivdarstellung einer Vorrichtung bei der Durchführung einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens; und

Figur 2

eine schematische Perspektivdarstellung einer Vorrichtung bei der Durchführung einer weiteren bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens.

The drawings show:

FIG. 1

a schematic perspective view of an apparatus in carrying out a preferred embodiment of the method according to the invention; and

FIG. 2

a schematic perspective view of a device in carrying out a further preferred embodiment of the method according to the invention.

FIG. 1 shows a schematic perspective view of an apparatus in carrying out a preferred embodiment of the method according to the invention. Shown is an object 1 with a curved surface 2. The surface is preferably curved in two spatial directions. On the surface, a section 3 can be seen. This section should be printed. For printing, an ink jet head 4 is provided. The head is shown in two positions, once as head 4a and once as head 4b. The head is moved along a first path A in a first direction A '. The web A (and also the web B, see below) is curved according to the surface and removed from the surface in such a way that high quality printing is possible and head-to-surface collision is prevented. During the movement along the first path, the head prints a first track a onto the surface. The first trace a (and also the web B, see below) consists of ink drops ejected from the nozzle by nozzles of a nozzle row. The ejection is controlled and takes into account both the movement of the head and the print image to be printed. The print image can be a solid or a grid. It may also contain eg text, image or pattern.
The ink-jet head 4 is also moved along a second path B in a second direction B '. The movements along both paths preferably take place by means of an articulated-arm robot, linear robot or a combined robot with rotary and sliding joints. The movements along both tracks can be done by moving the ink jet head or by moving the object or by a combination of both movements. During the movement along the second path, the head prints a second track b on the surface 2.
In FIG. 1 the first track edge a 'of the first track and the second track edge b' of the second track are shown. These meet at a point P and close an angle α where α is greater than 0 ° and less than 180 °, ie the two tracks are not parallel. In the example shown, the angle α is about 45 °.
In FIG. 1 It can be seen that the two webs A and B overlap in an overlap region 5. In the example shown, the two tracks intersect. It is also possible that the second web B only abuts the first web A, but is not continued on the opposite side of the first web. In the overlapping area, the inkjet head 4 preferably prints only on one of the two webs. In the example shown, the head only prints on the first web in the overlapping area. The first track a is therefore an uninterrupted track and the second track b is an interrupted track, ie the overlap area forms a gap in the second track. But it is also possible that is printed in one part of the overlap area on the first path and in the complementary part on the second path.
FIG. 1 also shows that there is at least one further overlap region 6 of the two webs A and B. The web B consists in this case of several track sections or a long, winding section which crosses the track A several times.
FIG. 2 shows a schematic perspective view of a device in carrying out a further preferred embodiment of the method according to the invention.
In FIG. 2 For example, the ink jet head 4 is again shown in two positions: once as the ink jet head 4a and once as the ink jet head 4b. Head 4a is substantially parallel to the direction A 'of the first web A with respect to its first orientation 7a and substantially perpendicular to the direction A' with respect to its second orientation 7b. The change in orientation is effected by rotating the head 4 about its axis 8 during forward movement, preferably by the robot. The respective orientation 7a and 7b of the head is parallel to the nozzle row of the head. The head prints the first track a during the forward motion. A correspondingly adapted rotation of the head also takes place on an adjacent second web B, on which the second track b is printed. The adaptation of the rotations (via a controller) takes place in such a way that the two tracks a and b vary in their respective width in the forward direction and their edges adjoin one another without gaps. In the example shown, the edges show a serpentine course. During the rotation of the head, its image data drive is to be varied in such a way that, despite the rotation and the resulting speeds and accelerations of the individual nozzles, a high-quality printing result is achieved.

LIST OF REFERENCE NUMBERS

1

object

2

surface

3

section

4

ink-jet head

4a

ink-jet head

4b

ink-jet head

5

overlap area

6

overlap area

7a

first orientation

7b

second alignment

8th

axis

A

first track

B

second track

A '

first direction

B '

second direction

a

first track

b

second lane

a '

first track edge

b '

second track edge

P

Point

α

angle

Claims (8)

1. Method for printing on at least a section (3) of a surface (2) of an object (1),

   - wherein an inkjet head (4, 4a, 4b) carries out a relative movement between the inkjet head (4, 4a, 4b) and the object(1) along a first path (A), thereby printing a first track, (a) and along a second path (B), thereby printing a second track (b),

   - wherein a first track edge (a') of the first track (a) and a second track edge (b') of the second track (b) meet at a point (P) and form an angle (a) of between approximately 1° and approximately 179° at the point (P),

   characterized in that

   - the second path (B) and the first path (A) intersect in multiple regions (5, 6) of overlap,

   - at least in a part of a region (5) of overlap, the inkjet head (4, 4a, 4b) only prints on the first path (A) or on the second path (B) or
   at least in a part of a region (5) of overlap, the inkjet head (4, 4a, 4b) prints on the first path (A) and in the complementary part, the inkjet head prints on the second path (B), and

   - the section (3) is composed of regions (5, 6) of overlap.
2. Method according to claim 1,
   characterized in
   that the angle (a) is between approximately 20° and approximately 70° or between approximately 110° and approximately 160°, preferably approximately 45° or approximately 135°.
3. Method according to any one of the preceding claims,
   characterized in
   that the inkjet head (4, 4a, 4b) changes its orientation (7a, 7b) relative to the respective path (A, B) on the first path (A) and/or on the second path (B).
4. Method according to any one of the preceding claims,
   characterized in
   that the angle (a) is selected in such a way that a=arctan (n*b/m*1) with α≠90°, wherein b=distance between two adjacent nozzles of an inkjet head (4, 4a, 4b), 1=distance between two successive print dots in the direction of movement of the inkjet head (4, 4a, 4b), and n, m=natural numbers.
5. Method according to any one of the preceding claims,
   characterized in
   that the width of the tracks (a, b) is modifiable, e.g. by rotating the inkjet head (4, 4a, 4b) or by switching off nozzles at the margin of the inkjet head (4, 4a, 4b).
6. Method according to any one of the preceding claims,
   characterized in
   that as soon as when the print data is created, i.e. at the RIP, the angles (α) used in path planning between the track edges (a', b') and the regions (5, 6) of overlap are already factored in in such a way that every image dot is only printed once even if the inkjet head (4, 4a, 4b) passes it multiple times.
7. Method according to claim 6,
   characterized in
   that print dots in the respective region of contact between two tracks (a, b) are assigned to one of the two tracks (a, b) on the RIP.
8. Method according to claim 7,
   characterized in
   that the track edges (a',b') are printed to fade out and mesh with neighboring track edges (a', b').

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